1. Field of the Invention
This invention relates to thermolytic distillation of the organic insulation material from metallic scrap to recover clean unoxidized metal and insulation by-products.
2. Description of the Prior Art
The history of the prior art in this area is aptly summarized in Urssing U.S. Pat. No. 3,877,474. Insulated electrical scrap such as wire, cable, magnetic coils, transformers and the like are utilized in large volumes in the electronics, electrical, communications, aerospace, and building industries, to name only a few. These materials typically comprise an inner metallic conductor encased within an outer sheath of insulating material. Multi-conductor cable, shielded wire and coaxial cable are commonly used variations which also contain metallic conductors and, in the case of shielded wire, metallic shielding. Copper is most frequently used as the inner conductor of wire and cable because of its excellent electrical conductive properties. However, other metals are also used such as, for example, aluminum. Typical insulating materials include paper, rubber, neoprene, polyvinyl chloride, polyurethane, polyethylene and Teflon.
Millions of pounds of scrap are produced annually in the United States and in other industrial nations from a number of sources. Firstly, as in most manufacturing processes, the fabrication of insulated wire and cable results in a certain amount of rejected scrap material. In addition, scrap is generated by the users of insulated wire and cable as a result of their fabrication processes, the scrapping of obsolete electrical and electronic equipment, replacement of power and telephone lines, the wrecking of buildings, and the rewiring of electrical and electronic systems and equipment. Generally, scrap insulated wire and cable is found in the form of masses of loose strands of varying lengths tangled into large coils or bales. The existence of tons of metal, such as copper, in this form has stimulated efforts to reclaim it economically.
In the prior art, a number of methods are known for the reclamation of metal from insulated scrap. One method of the prior art for reclaiming the metal conductors from scrap wire and cable is to remove the insulation manually by cutting and stripping means. Since this method requires each conductor wire to be treated individually, it is relatively slow and costly even on loose wire or cable. When the wire and cable is in the form of tangled coils or bales, the manual stripping method is prohibitive since the scrap would first have to be separated into individual strands prior to processing. In addition to being laborous and costly, this method is most suitable only with respect to wire and cable having larger diameters, such as AWG or MCM sized greater than 8.
Another method known in the prior art is to chemically dissolve the insulating material. This method has proved to be relatively uneconomical due to the typically high solids ratio of insulation to inner conductor by weight. For example, the insulation covering a copper inner conductor is typically 28 - 30 percent by weight of the wire. Because of the relatively high solids ratio, the cost of the chemical composition required to dissolve sufficient insulation to yield a pound of metal is too high with reference to the market value of the pound of metal yielded. An additional shortcoming of this method results from the fact that many different insulating materials are used in wires and cables. This fact requires the reclaimant to have various chemical compositions available, each one particularly suited to dissolving a particular type of insulating material.
Another common method of the prior art involves combustion of the insulating material to enable the recovery of the metal conductor within the wire or cable. This method has serious practical limitations. In the first place, the metal recovered is charred, i.e., oxidized due to the high temperature required to burn off the insulating material. In the case of copper conductors, combustion results in the loss of approximately 6 percent by weight of the copper which was potentially recoverable. Secondly, the oxidized metal is of a lower grade than unoxidized metal and therefore yields less on the market. An additional and very substantial shortcoming of the combustion method is that, without special purpose air pollution abatement equipment, it results in the emission of air pollutants into the air. Lastly, the burning method destroys the insulation and therefore whatever value the insulating material might have is lost.
In order to avoid the oxidation problems associated with the burning methods, the prior art has recently turned to heating in a non-oxidizing atmosphere. See, for example, Swartz, U.S. Pat. No. 3,821,026 and Deitz, U.S. Pat. No. 3,225,428. In another prior art process insulation decomposition is effected without wire oxidation by externally heating a reaction vessel which is sealed against the inflow of air, see O'Reilly, U.S. Pat. No. 3,448,509.